City Creek Nature Notes – Salt Lake City

January 9, 2017

January 9th

Filed under: Geology, Smells, Weather — canopus56 @ 10:09 pm

External Link to Image

Wasatch Mountain Range Front in Salt Lake Valley, Utah: Schematic of  Gross Geologic Structure. Not to scale. Looking east. After Stokes (1999) and Maebius (1999). T=Tertiary, J=Jurassic, Tr=Triassic, P=Permian, Pn=Pennsylvanian, M=Mississippian, C=Cambrian. Dotted line: Tertiary formation at south-west end of City Creek and Paleozoic Layers starting at north-east end of the canyon.

A Geologic “U”

4:00 p.m. It has rained heavily overnight with temperatures reaching into the fifties and in this unusual weather event, much of the snow in the lower canyon has been washed away. During my short jog in the canyon, the air during is pungent with new earth smells.

In addition to Pleasant Valley and Cemetery faults forming in the shape of a giant question mark (January 2nd), Van Horn and Crittenden’s 1987 geologic map also show how geologic layers in Salt Lake City are bent and tilted into a massive “U”-shaped synclinial structure (Stokes, 150; Van Horn and Crittenden 1987; Maebius 1999). Looking edge on to the Wasatch Front Mountain Range between Ensign Peak and City Creek to the north and Lone Peak to the south, flat layers of sediment are pinched by the up-ward push of East Canyon Fault and the Pre-Cambrian Farmington clayers on the north and the upward force of the intrusive granite of Little Cottonwood Canyon and Lone Peak on the south. A helpful visualization is to bend a few pieces of paper by holding the narrow ends of the sheets, and then look edge on at the long side. The north end of the valley is City Creek in your left hand and the south end of the valley is in your right hand. You are looking east towards Park City and Denver. Looking east at the Wasatch Front Mountain Range between Ensign Peak and City Creek to the north and Lone Peak to the south, flat layers of sediment are pinched by the up-ward push of East Canyon Fault and the Pre-Cambrian Farmington Complex layers on the north (Yonkee and Eaton 1999) and the upward force of the intrusive granite of Little Cottonwood Canyon and Lone Peak on the south (Stokes, 151, Fig. 15-14). The low point of this U-shaped bend is under the ridge between Emigration Canyon and Parley’s Canyon (id).

Originally, these layers where flat. In the 1600’s, Danish geologist Nicholas Steno first postulated the principle of original horizontality and the law of geologic superposition. The principle of horizontality assumes that geologic layers were originally deposited in flat layers, and the law of superposition assumes that newer geologic layers are laid down on top of older layers in the style of a tiramisu dessert. Other forces of nature then distort the original flat layers into the forms that we see today. Hintze and Kowallis and Yonkee and Easton summarize the total depth of the deposition layer below City Creek Canyon and the Wasatch Front. Adding the maximal depths for each of thirty-nine layers in Hintze, I find a maximum deposition of about 64,500 feet or about 12.1 miles (Hintze, 184, Stratigraphic Chart 34). This is about eleven times the height of Lone Peak above the Salt Lake Valley floor. Stokes roughly sketches the bottom of this great “U” shaped syncline at 10 miles beneath Parley’s canyon (Stokes, 151).

A consequence of this “U” shaped syncline is that the rocks of the same geologic age are expressed at the surface at each end of the “U”. Thus, six hundred and fifty million year old pre-Cambrian layers are found in both on the north in Farmington Canyon, Utah and again on the south end of the valley at the prominent north flank of Mt. Olympus and at the top of Big Cottonwood Twin Peaks at the south end of the valley (Yonkee and Eaton; Stokes, 151). These are followed by five-hundred and fifty year old Cambrian layers on the north side of City Creek Canyon (Sept. 1st; Stokes, 151; Van Horn and Crittenden; Maebius). Three-hundred and fifty million year-old Mississippian limestones are found on the north end of the valley at the south side of City Creek Canyon near Black Mountain and again on the south end of the valley at the north side of Neff’s Canyon. Two-hundred and twenty year old Triassic Thaynes Canyon Formation limestone can be found on the north end of the valley on the south side of Dry Fork Canyon and again on the south end of the valley beneath Grandeur Peak (Van Horn and Crittenden; Maebius). Two-hundred million year old Jurassic sandstone layers are found on the north at south massive of the Red Butte in Red Butte Canyon and on the south at the north side of Parley’s Canyon (Maebius).

At about 10 miles (15-16 kilometers) below the surface of the Front, the bottom of these “U” shaped brittle layers and the Wasatch Fault scrape along a ductile transition layer between supracrustal rocks and the North American continental plate (Hintze, 120, Fig. 155; Hintze, 125, Fig. 164); Stokes, 151, Fig. 15-14; Hintze, 68, Figs. 95-96). The North American Plate continues its expansion and drift to the east at the rate of 1-2 millmeters per year (Hintze, 120, Fig. 155). As a simple model, take a spoon in one-hand, and its curved bottom represents the faults along the Wasatch fault the merge under the western half of the valley. Represent the force of the continental plate by pushing with your index finger on the tip of the spoon. This forces the spoon up and toward the vertical. The same occurs with along the Wasatch Front, and the eastward force of the expanding continental plate generates a magnitude 7.0 earthquakes about every 6,000 years (Hintze, 126, Fig. 164). At one to two millimeters per year, 6 to 12 thousand millimeters represents about 20 to 40 feet of horizontal movement at the ductile transition layer 10 miles below the surface.

As a final model demonstration, fold your paper sheet representing the Wasatch Front along the long edge in four places, grab the ends and tilt the far long-edge of the paper above the front end. The left most fold, similar to the ruffle of a skirt, represents City Creek Canyon, one of the eight Wasatch Front Canyons in Salt Lake County. While thirty-five million year old Tertiary rocks are eroded away south of City Creek, in the canyon, a small “U”-shaped cup holds a remnant of Utah’s volcanic Tertiary past, the volcanic breccia found at canyon mile 1.1 near picnic site 7 (Jan. 7th). The right most edge of the sheets represents Lone Peak, the subsurface igneous intrusion also formed during the Tertiary (Ut. Geo. Survey, Pub. Info. 87; Hintze, 151).

In Thoreau’s “Journal” on January 9th, 1852, he again notes light blue shadows in snow hollows, and he notes that such shadows occur when falling snow filters out other colors of light. On January 9th, 1858, he comments on the various paths that falling snowflakes take to the ground.

On January 9th, 1918, City Water Commission C. Clarence Nelsen, who was later mayor of Salt Lake City from 1920 to 1928, proposed a reforestation plan for City Creek Canyon and the conversion of the canyon into a giant park (Salt Lake Telegram). Nelson proposed reforesting several thousand acres in Salt Lake City watershed canyons. Born in 1879, Nelson reminisced,

I can remember when in my boyhood days, City Creek canyon and other watersheds of the city were well timbered. The streams then delivered more water than they do now or at least their flow was more uniform. Since then however the trees have been burnt or cut away. Now is our opportunity to repair that damage. This generation may profit little from the reforestation but the next one will profit immensely and even a small tree will conserve some water. The reforestation will in time add much to the beauty of City creek (id).

See also, same, January 10th, 1918 (Salt Tribune and Salt Lake Herald). (See May 10th, 1918, Salt Lake Tribune, regarding the planting of 2,000 trees.) On January 9, 1913, the Commercial Club urged, due to a recent freeze that cut off all water to the City, that the City build a 5,000,000 gallon reservoir at Pleasant Valley in City Creek Canyon (Salt Lake Telegram). On January 9th, 1927, the 375 member Wasatch Mountain Club described its hikes sponsored during 1927, including a hike along the west ridge of City Creek Canyon (Salt Lake Telegram).

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